The atmosphere contains many particles, some of which are derived from anthropogenic sources. Those particles are comprised of various chemical and elemental constituents, including sulfates, nitrates, organic compounds and soot. Data on the chemical composition of airborne particles is needed to understand their origins and sources, and to evaluate the relationships between specific chemical constituents and potential environmental consequences.
With current technology, measurement of the concentration of chemical and elemental constituents of airborne particles involves several steps. First, a sample must be collected by pulling air through a filter or other particle collection device. At the end of the sampling period the particle filter or collection substrate must be removed from the sampler and transported to the laboratory. Finally, the substrate is analyzed for specific chemical species or elements using ion chromatography, X-ray fluorescence or other methods. That approach is the most widely used today. However, it is costly, and often results are not known until months after the sample was collected.
In contrast to particle monitoring, automated instruments are used routinely to measure concentration of specific gaseous pollutants. For ozone, nitrogen oxides and carbon monoxide automated monitors have made it possible to obtain real-time ambient concentration data at reasonable cost. They also provide immediate indications of pollutant levels. A need exists for equivalent, automated monitoring methods for the chemical and elemental constituents of airborne particles.
There are several automated monitors for particle mass available commercially. Those include beta-gauging methods (Macias and Husar, 1976; Dreiheller, Trost and Wendt, 1988; Wedding and Weigand, 1993), quartz crystal microbalance (Olin and Sem, 1971) and tapered element oscillating microbalance (Patashnick and Rupprecht, 1991). Although those instruments are used for monitoring, none provide data on the chemical or elemental composition of the airborne particles.
The older literature contains several references to automated particle monitors for particle sulfate (Coburn and Husar, 1978; Tanner, 1980; Huntzicker, 1986 and Allen, 1988). Those instruments use a flame photometric detector, and are not as stable as is desired for routine monitoring at typical ambient concentrations. To date, their use has been limited to research studies.
More recently, an automated instrument for measuring inorganic species in gases and particles has been reported by Simon and Dasgupta (1995). That instrument collects particles by steam injection and subsequent condensation, and then injects the condensate directly into an on-line ion chromatograph. That instrument is more complicated than the present invention, requiring handling of steam, liquid flows and continued operation of an ion chromatograph. Although the analytical capabilities of that instrument may be useful for research studies it does not seem likely that it will be suitable for routine monitoring applications.
The literature also describes an in-situ analyzer for carbonaceous aerosol (Turpin, Cary and Huntzicker, 1990). That instrument collects particles by filtration, then slowly heats the collected sample and quantitates the evolved carbon dioxide. With that instrument a second, backup filter is used to correct for adsorption of gaseous hydrocarbons onto the filter media. For reasons of cost and complexity, the use of that instrument has been limited.
Aerosol black carbon, operationally defined as optically absorbing particles, is measured by continuously monitoring the darkness of a filter deposit (Hansen, Rosen and Novakov, 1982, 1984). That approach is used in a commercial instrument called the Aethalometer.TM. (Magee Scientific, Berkeley, Calif.). The Aethalometer.TM. has been used for air monitoring studies, and provides high time resolution at low concentrations. However, the approach relies on a physical measurement unique to soot-like carbon. That approach cannot be extended to the monitoring of other particle species or elements.
An ambient carbon particulate monitor has been developed by Rupprecht and Patashnick Co. of Albany, N.Y. The monitor is an automated instrument design to measure the concentration of the carbonaceous fraction of airborne particles. It collects particles over a one hour time period, heats the collected sample and quantitates the evolved carbon dioxide using a non-dispersive infrared detector. A limitation is that the instrument collects particles using a conventional impactor which does not efficiently capture particles below 0.15 .mu.m in diameter. Soot-carbon particles found in the atmosphere are often below that size, and will not be captured. Additionally, that impactor has no provision for collection of solid particles. Because solid particles tend to rebound on the impactor, it is likely that those particles are not efficiently collected by that method. For those two reasons, that instrument may miss a significant portion of the particle carbon mass.
There is therefore a need for automated monitoring instruments for near real-time determination of the concentration of all the major chemical and elemental constituents of airborne particles.